I always go to the one just over the bridge on the right hand side (coming from the train station side) forget the name but its good on a sunny day.
Alternatively there is one right next to the cricket ground but that is normally packed.
If you get there early there are some great ones in town - Ye Olde Trip To Jerusalem, Bell Inn, The Malt Cross (My favourite - old victorian music hall) and there is the canal house near the train station.
I've got a brother and a brother-in-law in nottingham, thats why I know the establishments!
Shame I don't drink!